Classifications

• • C—CHEMISTRY; METALLURGY
  • C01—INORGANIC CHEMISTRY
  • C01C—AMMONIA; CYANOGEN; COMPOUNDS THEREOF
  • C01C1/00—Ammonia; Compounds thereof
  • C01C1/02—Preparation, purification or separation of ammonia
  • C01C1/04—Preparation of ammonia by synthesis in the gas phase
  • C01C1/0405—Preparation of ammonia by synthesis in the gas phase from N2 and H2 in presence of a catalyst
  • C01C1/0458—Separation of NH3
  • C01C1/0464—Separation of NH3 by absorption in liquids, e.g. water
• • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
  • Y02P—CLIMATE CHANGE MITIGATION TECHNOLOGIES IN THE PRODUCTION OR PROCESSING OF GOODS
  • Y02P20/00—Technologies relating to chemical industry
  • Y02P20/50—Improvements relating to the production of bulk chemicals
  • Y02P20/52—Improvements relating to the production of bulk chemicals using catalysts, e.g. selective catalysts

Definitions

• the present invention relates to a method for removal of ammonia from a gas mixture formed by catalytic production of ammonia.
• the gas mixture consists mainly of unconverted synthesis gas, some ammonia, inert gases and possibly also water. Ammonia and possibly also water are removed from the gas mixture by absorption with an organic absorption agent having two or more OH-groups, preferably a glycol.
• Ammonia is produced by catalytic reaction between nitrogen and hydrogen. Commonly used synthesis pressure is about 200 bar. The conversion is not complete and unconverted synthesis gas has to be recirculated to the reactor. The ammonia formed must accordingly be separated from the gas mixture. Some ammonia can be recirculated. The problem is to remove as much as possible of the ammonia in an economical way, and it is especially important to have an effective ammonia removal when the synthesis is carried out at relatively low pressure.
• Another problem is the water which gets into the gas mixture from earlier stages in the process, especially the methanation stage.
• the water in the synthesis gas will deactivate the catalyst. Accordingly, it is desirable to remove as much as possible of water which might be present before the gas reaches the synthesis reactor.
• Ammonia can be removed from said gas mixture in several ways. The most used one, especially in high pressure units, is to remove the ammonia from the synthesis gas by condensation with cooling water. However, only part of the ammonia will be removed in each recirculating step. If lower synthesis pressure is used, a cooling unit requiring high comsumption of energy to the compressor is used, and the costs of apparatus, for instance to heat exchangers will be high.
• Another method which has been proposed is to wash out the ammonia by water, for instance as described in GB 2.067.175 A.
• the ammonia can be separated from the water by destination.
• the main disadvantage of this method is that the gas mixture is moistened by water.
• the result of this is that the gas mixture has to be dried subsequent to the ammonia removal to avoid deactivation of the catalyst.
• a molecular sieve is used for removing water from the gas mixture.
• the object of the present invention was to arrive at an economical way of removing as much as possible of ammonia formed from unconverted synthesis gas in a gas mixture which shall be returned to the ammonia synthesis. It was especially desired to remove ammonia at conditions which make it possible to uitilize effectively cooling water for condensation of ammonia.
• the inventors aimed at purifying the gas mixture which should be returned to the ammonia synthesis to such a degree that the synthesis could be carried out at relatively low pressure and without substantial deactivation of the catalyst. In order to obtain this it was expected that the concentration of ammonia in the gas mixture should be below 0.5 volume!.
• the temperature at the bottom of the desorption column should be that high as present low energy heat from other parts of the ammonia units allows, for instance 100-150°C.
• the inventors then studied how to carry out the absorption and desorption with regard to utilization of cooling water. It was then found that by carrying out the absorption at a pressure being substantially the same as the pressure of the ammonia synthesis and then desorb ammonia from the desorption agent in at least two stages, one would be able to utilize cooling water in an economical way. It was then also possible to remove ammonia that efficiently that the gas mixture which should be returned to the synthesis contained less than 0.5 volume% ammonia. It was further found that it was most practical to desorb the main part of the ammonia in the first stage at 7-20 bar, condense with cooling water at a temperature of 5-35°C, then in a subsequent stage desorb ammonia at a pressure of 1- 3 bar. Energy could be saved by using further desorption stages at 3-15 bar.
• the water which can be present in the gas mixture originates mainly from the methanation step.
• This water can be removed in several ways known per se and at different places in the process. Thus it can for instance be removed ahead of the absorption column by means of glycol or from the absorption agent ahead of the desorption column. But it can also be removed from the purified extractant before it is returned to the absorption column.
• Fig. 1 shows a flow-sheet for removal of ammonia from a partly converted synthesis gas from an ammonia reactor.
• Fig. 2 shows a flow-sheet for removal of ammonia from a partly converted synthesis gas and where the ammonia is removed as a product at atmospheric pressure.
• Fig. 3 shows a flow-sheet for removal of ammonia from partly converted synthesis gas and where the ammonia is removed as a liquid product at cooling water temperature.
• Fig. 1 is shown the gas mixture 1 which shall be freed from as much as possible of its ammonia content and the purified synthesis gas mixture 17 which is transferred back to the ammonia reactor (not shown) .
• Completely regenerated absorption agent 21 is supplied to the absorption column 2 via pump 22, and partly regenerated absorption agent 3 via pump 14.
• the temperature in the column 2 is regulated by means of cooling water 4.
• the upper part 15 of the column is cooled by cooling water 16.
• the absorption agent containing absorbed ammonia is transferred out of the bottom of the column 2 through pipe 5 through the heat exchanger 6 and the pressure reduction valve 7 to the desorption column 8.
• Ammonia 11 is removed from the top of the column 8, while some of the ammonia condensed in cooler 10 where also some inert gas can be removed.
• Ammonia is transferred in reflux via pipe 12 back to the top of the column 2.
• the liquid from the column 8 is heated in heat exchanger 9 and the gas is returned to column 8.
• the liquid from the heat exchanger 9 is partly regenerated absorption agent 13.
• the main part of the absorption agent 13 is transferred in pipe 3 through heat exchanger 6 back to the absorption column 2.
• Some of the partly regenerated absorption agent 13 is trans ⁇ ferred through pipe 18 and reduction valve 19 to a separating tank 20.
• the absorption agent is regenerated there.
• the regenerated absorption agent 21 is returned to the absorption column 2.
• the desorbed ammonia 23 from the separation tank 20 is first cooled by cooling water 25 in the cooler 24. Condensed absorption agent 26 is transferred to pipe 21.
• Desorbed ammonia 23 is compressed in compressor 27 and transferred to the desorption column 8.
• Fig. 2 shows an embodiment of the invention where ammonia finally is removed as liquid at atmospheric pressure.
• Ammonia from the top of the desorption column 8 is cooled and trans ⁇ ferred by pipe 11 via pressure relieve valve 28 to a separation tank 29.
• the liquid ammonia product 30 is removed at the bottom of the separation tank 29.
• Gaseous ammonia from the separation tank 29 is transferred in pipe 31 to heat exchanger 32 to the compressor 27.
• Non-condensed gas 33 from the cooler 10 is cooled in cooler 32 and transferred to separation tank 34.
• Inert gases are removed through pipe 35 and condensed liquid is transferred in pipe 36 to a reduction valve 37 to the separa ⁇ tion tank 29. The remaining part of the process is as shown in Fig. 1.
• Fig. 3 shows an alternative way of treating desorbed ammonia from separation tank 20.
• Ammonia is transferred to the absorp ⁇ tion tank 26 and cooled by cooling water 27.
• the absorption agent is supplied through pipe 24 and is cooled in cooler 25.
• the liquid 28 from the absorption tank 26 is pumped back to the desorption tower 8 via pump 29 and heat exchanger 25.
• This example shows the method according to the invention carried out in a unit as described in Fig. l.
• Gas mixture 1 at 25°C, 50 bar and containing 10 volume! NH 3 is contacted by the ethylene glycol containing some ammonia in column 2.
• Regenerated absorption agent (0.8 weight! NH 3 ) 22 is supplied to the top of the column, and partly regenerated absorption agent 3 (6.8 weight! NH 3 ) is supplied at the centre of column 2.
• Cooling water 4 and 16 secure that the temperature in the absorption tower is kept at about 25°C.
• the ammonia content of the liquid at the bottom of the tower 5 became about 16 weight!.
• the stream 5 from column 2 is warmed in heat exchanger 6, and the pressure is relieved in valve 7 to 11 bar and then trans ⁇ ferred to absorption column 8. Liquid at the bottom of the column is heated in the boiler 9 at about 130°C. Ammonia at the top of the desorption column 8 is condensed in condenser 10 at about 25°C. Inert gas can also be removed in the condenser. Some of the ammonia is returned to the absorption column as reflux and the rest is product ammonia at 25°C.
• the liquid stream 13 from the boiler 9 contains 6.8 weight! ammonia.
• the stream 3 is cooled in the heat exchanger 6 and pumped by pump 14 back to the centre of the absorption column
• the liquid stream 18 from the boiler is pressure relieved by valve 19 to 1 bar and transferred to the desorption tank 20.
• Ammonia at the top of the desorption tank 23 is cooled in cooler 24 at about 25°C, compressed to 11 bar and transferred to desorption column 8.
• Condensate 26 from cooler 24 is transferred together with the liquid stream from the desorption tank 21.
• Liquid from the desorption tank 20 containing about 0.8 weight! ammonia is pumped by the pump 22 to the top of the absorption colum 2.
• the ammonia concentration at the top of the absorption column becomes 0.5 volume!. This means that about 95! of the ammonia in the stream 1 is absorbed and is removed as product 11.
• This example shows the method according to the invention carried out in a unit as described in Fig. 2.
• the liquid ammonia stream 11 from Example 1 is at 25°C.
• the pressure of the stream 11 is reduced to 1 bar in valve 21 and the liquid is transferred to separation tank 29.
• the temperature in the tank 29 will then be -33°C.
• the ammonia gas from the separation tank 29 is heated in heat exchanger 32 and transferred together with the stream 23 to the compressor 27.
• Non-condensed gas from cooler 10 is cooled in the heat ex ⁇ changer 32 and transferred to separation tank 34.
• the gas stream 35 from the separation tank 34 contains minor amounts of nitrogen, methane and argon.
• Condensed liquid from separation tank 34 is expanded in the valve 37 and transferred to separa ⁇ tion tank 29.
• the ammonia product 30 will be a liquid at -33°C.
• the ammonia concentration in the gas mixture returned to the synthesis was 0.5 volume!. This means that about 95! of the ammonia in the gas mixture from the synthesis was absorbed and removed as a product 30.
• This Example shows the method according to the invention carried out in a unit as described in Fig. 3. The difference between this method and that of Example 1 is found in the way the ammonia gas is transferred back to the process.
• the method is the same as in Example 1 except that the boiler 9 is operated at about 105°C such that partly regenerated diethylene glycol contains about 5.3 weight! NH 3 .
• the ammonia gas 23 is transferred to the absorption tank 26.
• Partly regenerated diethylene glycol 24 is cooled in the heat exchanger 25 and transferred to the absorp ⁇ tion tank 26.
• the absorption tank 26 is cooled by cooling water 27 such the temperature is kept at about 25°C.
• Liquid 28 from the absorption tank 26 containing about 8.5 weight! NH 3 is pumped up to 11 bar by pump 29, heated in the heat exchanger 25 and transferred to the absorption column 8.
• a by-effect of the method according to the invention is that processing of the purged gas (inert gas removal) can be carried out in a simpler way than usual because the ammonia content is low.
• the method will further be suitable if it is desired to bring the water content below 1 ppm in the gas mixture before it is returned to the ammonia synthesis.

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• Chemical & Material Sciences (AREA)
• Organic Chemistry (AREA)
• Chemical Kinetics & Catalysis (AREA)
• Analytical Chemistry (AREA)
• Inorganic Chemistry (AREA)
• Organic Low-Molecular-Weight Compounds And Preparation Thereof (AREA)
• Treating Waste Gases (AREA)
• Exhaust Gas Treatment By Means Of Catalyst (AREA)
• Gas Separation By Absorption (AREA)
• Incineration Of Waste (AREA)

Priority Applications (3)

Applications Claiming Priority (2)

Publications (1)

Family

ID=19891699

Family Applications (1)

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Citations (1)

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• 1989
  • 1989-02-03 NO NO890439A patent/NO167082C/no unknown
• 1990
  • 1990-02-02 AT AT90902848T patent/ATE113569T1/de not_active IP Right Cessation
  • 1990-02-02 US US07/688,592 patent/US5230877A/en not_active Expired - Fee Related
  • 1990-02-02 AU AU50414/90A patent/AU5041490A/en not_active Abandoned
  • 1990-02-02 CA CA002045674A patent/CA2045674A1/en not_active Abandoned
  • 1990-02-02 EP EP90902848A patent/EP0460001B1/de not_active Expired - Lifetime
  • 1990-02-02 DE DE69013898T patent/DE69013898D1/de not_active Expired - Lifetime
  • 1990-02-02 WO PCT/NO1990/000028 patent/WO1990008736A1/en active IP Right Grant

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